In completing a well, various operations are performed in the wellbore, including operations in which explosive devices are detonated. Examples of explosive devices include perforating guns, pipe cutters, tools for setting packers, and so forth.
Activating an explosive device in a wellbore relies on the fault-free operation of a relatively complex collection of individual subsystems. While each subsystem has been designed to achieve a target reliability level, the collection of the individual subsystems may produce an unacceptably high system failure rate. In particular, the electrical transmission path (from the earth surface down to the explosive device located downhole in the wellbore) presents particular difficulties, as failure mechanisms can be difficult to isolate, leading to multiple failed attempts at activating the explosive devices before the root cause is isolated and resolved. This problem is especially acute in the case of intermittent failures (such as due to short circuits), which may be present while the equipment is deployed downhole, but then disappear when the tools are brought to the more benign conditions of the earth surface for troubleshooting. Equipment may often be replaced and classified as defective unnecessarily when the fault disappears for an unrelated reason.
There are two fundamental approaches to monitoring the integrity of an electrical circuit during operations involving activation of explosive devices: (1) surface testing and (2) downhole testing. Surface testing involves testing the integrity of the system at the surface before deployment in the well, or possibly before redeployment if the equipment has been recovered for diagnostics as a result of a failure. Surface testing involves testing the electrical continuity or insulation integrity of specific subsystems (e.g., wireline, casing collar locator, firing head, and so forth). To perform a thorough system test, shooting power may sometimes be applied (shooting power refers to power that is at a sufficiently high level to activate the explosive device). However, performing such a test at the earth surface is hazardous due to possible inadvertent detonation of the explosive device at the earth surface.
Downhole testing often relies upon sophisticated testing equipment that are coupled to but are separate from the explosive device. However, such relatively sophisticated equipment are associated with relatively high costs that may not be practical in many situations.